A blade or vane of this type is known for example from U.S. Pat. No. 4,278,400.
Modern high-efficiency gas turbines use blades or vanes which are provided with a cover strip and, during operation, are exposed to hot gases at temperatures of more than 1200 K and pressures of more than 6 bar.
FIG. 1 illustrates a basic configuration of a blade or vane with cover strip of this type. The blade or vane 10 comprises a main blade or vane part 11 which toward the bottom merges via a blade or vane shank 25 into a blade or vane root 12. At the upper end, the main blade or vane part 11 merges into a cover-strip section 21, which, in a complete ring of blades or vanes, together with the cover-strip sections of the other blades or vanes, forms a continuous, annular cover strip. The main blade or vane part 11 has a leading edge 19, onto which the hot gas flows, and a trailing edge 20. A plurality of radial cooling ducts 13, 14 and 15, which are connected to one another in terms of flow by diverter regions 17, 18 and form a serpentine with a plurality of turns, are arranged in the interior of the main blade or vane part 11 (cf. the flow arrows in the cooling ducts 13, 14, 15 in FIG. 1).
On account of the single passage of the cooling medium through the cooling ducts 13, 14, 15 which are connected in series in the form of a serpentine, the temperature of the cooling medium increases as it flows through the cooling ducts, reaching a maximum in the final cooling duct 15 of the trailing edge 20. Therefore, under certain operating conditions the trailing edge 20 of the blade or vane 10 may reach excessively high temperatures in terms of the cooling medium and the blade or vane material or metal. The resulting mismatch of the metal temperature over the axial length of the blade or vane may lead to high-temperature creep and consequently to deformation of the trailing edge 20. A secondary effect of the trailing-edge deformation for a blade or vane with cover strip as shown in FIG. 1 is tilting of the cover-strip segments 21 in the axial, radial and circumferential directions. The tilting of the cover-strip segments 21 can lead to the gaps between individual cover-strip segments opening up, allowing high-temperature hot gas to enter the cover-strip cavity. This can significantly increase the temperatures of the cover-strip metal and can rapidly give rise to creeping of the cover strip and ultimately can lead to high-temperature failure of the cover strip.
Document U.S. Pat. No. 4,278,400, which was mentioned in the introduction, has already proposed a multiple supply of medium for cooling blades or vanes with a cooled tip and finely distributed cooling openings at the leading edge (film cooling). An ejector is arranged transversely to the direction of flow of the main cooling stream at the end of a 90° diversion of the main cooling stream, which injector injects an additional stream of cooler cooling medium into the cooling duct running along the trailing edge. The ejector is supplied with cooling medium via a duct running radially through the root. The cooling medium which flows out of the nozzle of the ejector at an increased velocity generates a reduced pressure, which draws the heated cooling medium out of the cooling duct of the leading edge into the cooling duct of the trailing edge. Approximately 45% of the cooling medium flowing along the leading edge emerges through the cooling openings at the leading edge. 40% is sucked in by the injector. The remainder is discharged through cooling openings at the blade or vane tip.
This known way of effecting multiple supply of cooling medium has various drawbacks: the injector hugely changes the pressure conditions and flow conditions in the cooling ducts compared to the configuration with a single supply through the inlet of the cooling duct at the leading edge. In particular, it is necessary to find an equilibrium between the cooling medium flowing out for film cooling at the leading edge and the cooling medium sucked in by the injector and then to set this equilibrium. This requires a completely new design of the blade or vane cooling, which can only be adapted to changing requirements with difficulty. The injector principle and the associated reduced-pressure generation are unsuitable for blades or vanes without film cooling of the leading edge and blades or vanes with a cooled cover strip.